North Korea is days away from the launch of a long-range missile, United States officials have said, heightening concerns about the risks to regional stability.

Counter-proliferation and intelligence officials said the country had mounted a rocket on a launch pad and placed together two stages of what will probably be a three-stage device, Reuters reported today. It could be ready to fire as early as the weekend, sooner than expected.

The US secretary of state, Hillary Clinton, warned that firing a long-range rocket would be a "provocative act" that would have consequences, while South Korea said it would be "a serious challenge and provocation", threatening regional stability.

Pyongyang has said it plans to launch a communications satellite between 4 and 8 April. But Washington, Seoul and Tokyo accuse it of planning a weapons test in breach of a United Nations security council ban. The launch technology for either scenario is nearly identical.

While relations between North and South Korea have deteriorated dramatically since President Lee Myung-bak came to power in Seoul last year and took a tougher stance towards his neighbour, Pyongyang has appeared to be seeking improved relations with the new US administration, drawing back from the denunciations it directed at the former president George Bush.

South Korea's Chosun Ilbo newspaper said today that US spy satellites detected the rocket on the launch pad at the north-eastern Musudan-ri missile base two days ago.

"Technically a launch is possible within three to four days," an unnamed diplomatic source in Seoul told the publication.

Seoul's defence ministry said it could not confirm the rocket's placement.

But spokesman Won Tae-jae said: "We are strongly urging the north to immediately halt [preparations]."

Experts believe it is a Taepodong-2 – the north's longest range missile. In theory it could carry a warhead as far as Alaska, although a previous test flight failed within seconds of the launch in 2006.

A successful test this time would boost domestic morale and be seen as a humiliation for the south, which hopes to launch its own satellite later this year.

Pyongyang's official Korean Central News Agency said Seoul was trying to stop the launch because "it can only be a serious blow to the Lee group, which has been trying to suffocate its compatriots".

North Korea has warned international agencies of the rocket's planned trajectory, which would take it over Japan, dropping booster stages to the east and west.

Officials in northern Japanese coastal cities today began setting up emergency networks and running drills to prepare for falling debris in case the launch fails. The government is preparing an order to deploy a missile shield, including interceptors, to protect against debris.

Admiral Timothy Keating, the head of US Pacific Command, has said the US military could, with "high probability", intercept any missile heading for American territory if ordered to do so.

But Pyongyang has said any attempt to shoot down its "for peaceful purpose" satellite would be an act of war and has warned it would quit the stalled six-party aid-for-nuclear-disarmament talks.

Discord on the peninsula has been exacerbated by the annual joint military drill by South Korean and US troops, which has been larger than usual.

The situation may be further complicated by the north's detention of two US journalists, who were arrested last week on the Chinese border. They are reportedly under interrogation in Pyongyang.